Would a 6Ω secondary be OK for 4-8Ω speakers?
The speakers I will be building for this amp are 6Ω but for future purposes I'm just wondering.

Use Google Translate - we discussed this matter in that topic. It is not about speaker, it is about optimum load of the tube. Search here on the forum for load lines. GXSE10-6-10K is what doctor prescribed for Frank's schematic. However, for better bass I would use 25W Edcor transformer instead of 10K one.